A nickel-cadmium storage battery and nickel-metal hydride battery are well known as batteries having high capacity. The nickel-metal hydride battery is a storage battery having a negative electrode including a hydrogen absorbing alloy capable of absorbing and releasing hydrogen. It is excellent environmentally and has been widely used as a sealed miniature (small-sized) secondary battery.
In the nickel-metal hydride battery, the hydrogen absorbing alloy is used as an active material for the negative electrode. A rare-earth hydrogen absorbing alloy having a crystal structure of the CaCu5 type has been used as the hydrogen absorbing alloy.
As a hydrogen absorbing alloy having a greater capacity than the rare-earth hydrogen absorbing alloy having a crystal structure of the CaCu5 type, a hydrogen absorbing alloy comprising a rare-earth, Mg, and Ni as main components has been proposed. As such hydrogen absorbing alloy, a hydrogen absorbing alloy containing 10˜50 atom % of La as a rare-earth element has been proposed in Japanese patent laid-open publication No. 2001-316744.
However, the hydrogen absorbing alloy disclosed in the Japanese patent publication identified above cannot provide a nickel-metal hydride battery having a sufficient life cycle.